Multi-projection systems (hereafter MP systems) are increasingly used, in particular in contexts where a very large display or projection area is required, for instance in a dome, a stadium or a concert hall, or for projection on buildings.
In practice, an MP system comprises an array of display devices, for instance of video projectors (VP) that each projects or displays a part (sub-image) of an image on a screen.
Usually, the different VPs cover adjacent, partially overlapping zones of the total screen area in order to ensure a smooth transition between different projected sub-images and provide a tolerance against small displacements which may be introduced, for example, by vibrations or by thermal expansion.
To that end, a blending process is performed to generate adapted overlapping zones. In practice, the blending process for a sub-image consists in duplicating pixels of edge areas of the surrounding sub-images of the considered sub-image.
The definition and the size of a projected image depend on the focal length of the VP lens, the size of the VP's light modulation device (e.g. an LCD panel) and the distance between the VP and the screen or display zone.
VPs are commonly equipped with zoom lenses (i.e. lenses with variable focal length) in order to provide the user with freedom to adapt VP installations to given spatial constraints, for example to select the distance between the VP and the screen.
Document US 2008/036971 proposes an MP system that aims at simplifying the installation setup based on the exchange of information relative to the composite display scheme between the VPs.
A transmission/reception section associated with a configuration is used, permitting for each VP to determine a specific cutting out of the original image to be displayed by the display device.
However, in this solution, each VP has to support the maximum video resolution of the original image. Consequently, the VP input interface depends on the video source resolution, thus making the composite display not really scalable.
Other solutions are known wherein different video streams, each comprising only the sub-image (and not the whole image) and the overlapping zones to be displayed by a corresponding VP are transmitted to each VP.
However, at the source server, the complexity of this solution increases with the number of VPs, since the same number of applications of the blending process are required. This solution thus also requires interfaces with a large bandwidth to send the sub-images together with the duplicated pixels.
There is a need to improve known multi-projection systems to allow an improved scalability and reduce the bandwidth requirements of the source server and of the display devices.